docs: def export_losses_by_asset_npz(ekey, dstore):
      """
      :param ekey: export key, i.e. a pair (datastore key, fmt)
      :param dstore: datastore object
      """
      fname = dstore.export_path('%s.%s' % ekey)
      savez(fname, **dict(extract(dstore, 'losses_by_asset')))
      return [fname]